Turkish-backed Syrian forces have reinforced the area around the town of Manbij, a rebel spokesman said on Monday, as they prepare for a withdrawal of US troops following Washington’s surprise decision to pull out of Syria.

The heightened military activity comes as Ankara and Washington have agreed to coordinate on the US withdrawal. US President Donald Trump’s abrupt decision has upended American policy in the region and set Turkey up for a broad push against Syrian Kurdish YPG forces across its border.

“Yesterday units from the Syrian National Army headed towards the Manbij front and took preliminary positions (in preparation) for the battle,” said Major Youssef Hamoud, the spokesman for the National Army, the main Turkey-backed rebel force in the area. The group is aimed at unifying disparate factions in northwest Syria.

Manbij has been a major flashpoint between Ankara and Washington. In June, the NATO allies reached an agreement that would see the YPG ousted from the town but Turkey has said the deal has been delayed.

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President Recep Tayyip Erdogan said on Dec. 14 that Turkish forces would enter the town if the United States does not remove the Kurdish fighters.

US forces still remain in Manbij, and the Turkey-backed fighters will not advance until they withdraw, Hamoud told Reuters.

The YPG has been the main US ally in the fight against Islamic State in Syria. Ankara says the YPG is a terrorist organization and an extension of the Kurdistan Workers Party (PKK), which has waged an insurgency in Turkey since the 1980s.

Erdogan has said Ankara will postpone a planned military operation against the YPG east of the Euphrates river in northern Syria following the US decision to pull out.

Erdogan and Trump agreed in a phone call on Sunday to coordinate to prevent an authority vacuum from developing as the United States withdraws from Syria, the Turkish presidency said.

On Sunday, a Reuters witness saw hundreds of vehicles head to the southern border province of Kilis after leaving Hatay, another border province. The convoy included tanks, howitzers, machine guns and buses carrying commandos, the witness said.